Fuser thermal protector

ABSTRACT

An apparatus arranged to cooperate with a fusing device for preventing the occurrence of an over-temperature condition therein.

PATENTEUUEEZSESYS SHEEI 1 [IF 4 PATENIEUUECZM StEEIJnfq WIN;

BACKGROUND OF THE INVENTION This invention relates generally to an electrostatographic printing machine, and more particularly concerns an apparatus arranged to tie-energize the printing machine when an over-temperature condition occurs in the fusing device thereof.

In general, electrostatographic printing includes electrophotographic printing and electrographic printing. An electrophotographic printing machine charges a photoconductive surface to a substantially uniform level, and, thereafter, projects a light image thereon. Thelight image selectively discharges the charged photoconductive surface to record thereon an electrostatic and fused thereto. Generally, the toner particles are a 1 heat settable colored thermoplastic material. A suitable fuser positioned appropriately affixes the powder image to the copy sheet. Electrographic printing differs from electrophotographic printing in that the electrostatic latent image is formed without the necessity of creating a light image of the original document to be reproduced.

Numerous differing types of fusing devices may be utilized in electrostatographic printing machines. For example, many fusing devices utilize radiant elements for heating the toner material to the set point thereof. Other types of fusing devices heat the conveyor belt on which the copy sheet is transported. Finally, some fusing devices incorporate both of the foregoing features, i.e., the fuser employs radiant elements for heating the toner material and a heated conveyor belt to insure that the copy sheet functions as a heat source rather than a heat sink.

A problem common to all of the foregoing types of fusing devices, as well as others not heretofore discussed, is that the temperature thereof must be controlled to insure that excess heat is not generated therein. If the temperature produced in the fusing device is excessive, the conveyor belt may be destroyed or the copy sheet having the toner powder image thereon may exceed its kindling temperature producing a fire therein. Generally, in electrophotographic printing machines, the conveyor belt is an elastomeric material and the copy sheet material is paper. Hence, the various fusing devices incorporate fail-safe devices to prevent the occurrence of a catastrophic failure. It, therefore, becomes of paramount importance to ascertain the presence of an over-temperature condition within the fusing device. Upon the occurrence of such a condition, the fusing device should be deactuated.

2 Accordingly, it is a primary object of the present invention to improve the apparatus utilized for thermal protection of a fusing device employed in an electrostatographic printing machine.

SUMMARY OF THE INVENTION Briefly stated, and in accordance with the present invention, there is provided an apparatus cooperating with a fusing device for preventing an overtemperature condition from developing therein.

In accordance with the present invention, the fusing device includes a passageway adapted to facilitate the movement of an article therethrough. Preferably, means are provided for transporting the article through the passageway in the fusing device. Detecting means contact the surface of the transport means opposed from the article supporting surface thereof. In this manner, the detecting means is arranged to indicate an over-temperature condition in the fusing device by sensing the occurence of a heat blister in the transport means.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in'which:

FIG. 1 is a schematic perspective view of an electrophotographic printing machine embodying the features of the present invention therein;

FIG. 2 is a perspective view of the fusing device utilized in the FIG. 1 printing machine;

FIG. 3 is a plan view of the FIG. 2 fusing device lower housing;

FIG. 4 is a sectional elevational'view taken along the line 4-4 of FIG. 3 in the direction of the arrows;

FIG. 5a is a fragmentary, perspective view of the apparatus of the present invention depicted in the closed position to maintain the FIG. 2 fusing device in the energized state; and

FIG. 5b is a fragmentary, perspective view of the F lG.5a apparatus shown in the opened position sensing an over-temperature condition in the fusing device.

While the present invention will be described in connection with the preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION With continued reference to the drawings, FIG. 1 schematically illustrates a multi-color electrophotographic printing machine. The electrophotographic printing machine depicted therein includes a fusing device arranged to coalesce a multi-layered toner powder image to a sheet of support material. Although the thermal protector of the present invention is particularly well adapted for use in the fusing device of the electrophotographic printing machine depicted in FIG. 1, it should become evident from the following description that it is equally well suited for use in a wide variety of fusing devices and is not necessarily limited in its application to the particular embodiment shown herein.

Referring once again to FIG. 1, the printing machine employs a photoconductive member having a rotatably mounted drum with a photoconductive surface 12 secured to the exterior circumferential surface thereof. Drum 10 is driven by the printing machine main drive motor (not shown) in the direction indicated by arrow 14. In this way, as drum 10 rotates, photoconductive surface 12 passes sequentially through a series of processing stations. The main drive motor is arranged to rotate drum 10 at a predetermined speed relative to the other operating mechanisms of the printing machine. Thus, the various machine operations are coordinated with one another to produce the proper sequence of events at the appropriate processing stations.

As drum 10 rotates in the direction of arrow 14, photoconductive surface 12 first passes through charging station A. A corona generating device, indicated generally by the reference numeral 16, is positioned at charging station A. Preferably, corona generating device 16 extends in a longitudinal direction transversely across photoconductive surface 12. A suitable corona generating device is described in U.S. Pat. No. 2,778,946 issued to Mayo in l957.Corona generating device 16 is adapted to produce a spray of ions for charging photoconductive surface 13 to a relatively, high substantially f lter mechanism is described in copending application Ser. No. 830,282 filed in 1969. Turning once again to FIG. 1, an original document 22, such as a sheet of paper, book, or the like is disposed face down upon transparent platen 24. A lamp assembly, indicated generally at 26, is operatively associated with lens system 18 and filter mechanism 20. Lamp assembly 26, lens system 18 and filter mechanism are moved in a timed relation with drum 10 to scan successive incremental areas of original document 22 disposed upon platen 24. The

' foregoing arrangement produces a flowing light image of original document 22 which is projected onto charged photoconductive surface 12. The irradiated areas of photoconductive surface 12 are discharged. In this manner, an electrostatic latent image corresponding to original document 22 is created on photoconductive surface 12. As hereinbefore indicated, filter mechanism 20 is adapted to interpose selected color filters into the optical light path. The appropriate color filter operates on the light rays passing through lens 18 to record an electrostatic latent image on photoconductive surface 12 corresponding to a preselected spectral region of the electromagnetic wave spectrum, hereinafter referred to as a single color electrostatic latent image.

After the single color electrostatic latent image is recorded on photoconductive surface 12, drum l0 rotates to development station C. At development station C, three individual developer units, generally indicated by the reference numerals 28, 30 and 32, respectively, are adapted to deposit toner particles on the electrostatic latent image, thereby forming a viewable powder image. One type of development station employing a plurality of suitable developer units is disclosed in copending application Ser. No. 255,259 filed in 1972.

Preferably, the developer units are all of a type referred to generally as magnetic brush developer units. Typical magnetic brush developer units use a magnetizable developer mix having carrier granules and toner particles therein. The toner particles are generally heat settable. Development of the electrostatic latent image recorded on photoconductive surface 12 is achieved by contacting it with a brush of developer mix. Each of the respective developer units 28, 30 and 32, are arranged to apply toner particles corresponding to the complement of the color separated latent image recorded on photoconductive surface 12. For example, developer unit 28 deposits cyan toner particles on a red filtered latent image, developer unit 30 deposits magenta toner particles on a green filtered latent image, and developer unit 32 deposits yellow toner particles on a blue filtered latent image. The aforementioned steps of developing various toner powder images on the respective single color electrostatic latent image occurs sequentially rather than simultaneously in the embodiment of the electrophotographic printing machine disclosed herein.

After the single color electrostatic latent image has been developed with the'appropriately colored toner particles, drum 10 rotates to transfer station D. At transfer station D, the toner powder image adhering electrostatically to photoconductive surface 12 is transferred to a copy sheet or sheet of final support material 34. Final support material 34 may be, amongst others, plain paper or a sheet of polysulphone thermoplastic material. Support material 34 is secured releasably to a transfer roll, indicated generally by the reference numeral 36. Transfer roll 36 is adapted to rotate in-the direction of arrow 38 in synchronism with drum 10 (in this case at the same angular velocity therewith). U.S. Pat. No. 3,612,677 issued to Lang'don et al. in 1972 discloses a suitable electrically biased transfer roll. Transfer roll 36 is biased electrically to a potential such that the magnitude and polarity thereof is sufficient to attract electrostatically the toner powder image from photoconductive surface 12 to support material 34. Inasmuch as transfer roll 36 is adapted to rotate at the same angular velocity as drum l0, successive toner powder images may be transferred to support material 34 in superimposed registration with one another. Thus, a mu'lti-layered toner powder image is formed on support material 34 which corresponds to the multicolor original document being reproduced. After the initial toner powder image is transferred to support material 34, the aforementioned steps of charging, exposing, developing and transferring are repeated for the.

next successive color separated light image. Hence, after a plurality of powder images are transferred to support material 34, a composite picture of the original is created thereon.

As shown in FIG. 1, gripper fingers 40 secure support material 34 releasably to bias transfer roll 36. After the last powder image has been transferred to support material 34, gripper fingers 40 space support material 34 from transfer roll 36. After gripper fingers 40 have spaced support material 34 from transfer roll 36, stripper bar 42 separates support material 34 from transfer roll 36.

Prior to proceeding with the description of the formation of the multi-color copy, the sheet feeding apparatus employed herein will be briefly described. A stack 43 of support material is supported on tray 44. A feed roll 46, cooperating with a retard roll 48, advances and separates the uppermost sheet of support material from stack 43. The advancing uppermost sheet of support material enters into chute 50. Chute 50 guides the advancing uppermost sheet of support material into the nip between register rolls 52. Register rolls 52 align the sheet of support material and forward it in properly timed sequence to transfer roll 36 so as to be secured thereto by gripper fingers 40. As hereinbefore described, after the last toner powder image has been transferred to support material 34, gripper fingers 40 space support material 34 from transfer roll 36 and stripper bar 42 is interposed therebetween to separate it therefrom.

Conveyor 54 advances support material 34 from transfer roll 36 to fusing station E. At fusing station E, a suitable fusing device, indicated generally at 56, af-

fixes the multi-layered toner powder image to support 'material 34. A suitable fusing device 56 is described in copending application Ser. No. 300,531, filed in 1972. As shown in FIG. 1, the apparatus of the present invention is'incorporated in fusing device of 56. Hence, fusing device 56 includes a thermal protector, indicated generally at 58, which is adapted to prevent fuser 56 from reaching a temperature condition endangering belt 88 and the support material passing therethrough. Fusing device 56, in conjunction with thermal protector 58 will be described hereinafter in greater detail with refernces to FIG. 2 through 5b, inclusive. After the fusing process, sheet 34 is advanced by endless belt conveyors 62 and 64 to catch tray 66 for subsequent removal therefrom by the machine operator.

Although a preponderance of the toner particles are transferred to support material 34, invariably residual toner particles remain on photoconductive surface 12. Residual toner particles are arranged to be removed from photoconductive surface 12 as drum moves through cleaning station F. At cleaning station F, the residual toner particles are initially brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining thereon. Thereafter, the neutralized toner particles are mechanically cleaned from photoconductive surface 12 by a rotatably mounted fibrous brush 68. One type of suitable brush cleaning arrangement is described in US. Pat. No. 3,590,4l2 issued to Gerbasi in 1971. As shown in FIG. 1, a rotatably mounted brush 68 is disposed at cleaning station F in substantial contact with photoconductive surface 12. Thus, residual toner particles remaining on photoconductive surface 12, after each successive transfer operation, are readily removed therefrom.

It is believed that the foregoing description is suffi cient'for purposes of the present application to illustratethe general operation of a multi-color electrophotographic printing machine embodying the teachings of the present invention therein.

depicted as being pivoted to an open position to more clearly illustrate the various components thereof. Conveyor 54 is associated with fusing device 56 so as to transport support material 34 from transfer roller 36 thereto. As illustrated in FIG. 2, conveyor 54 includes a plurality of endless belts 70 entrained about a pair of opposed, spaced rollers 72. A vacuum system maintains a low pressure by drawing air through apertures 74 in belt 70 to tack support material 34 thereto. Cover member 76 has a radiant energy source or heat strips 80 preferably made from a nickel chromium alloy ribbon entrained helically between a pair of spaced, opposed ceramic spool supports. One of the ceramic spools is mounted on a leaf spring permitting compensation for the thermal expansion and contraction of the nichrome ribbon. Heat strips 80 are configured to provide substantially uniform radiation, the end elements thereof being arranged to minimize radiation fall-off. A pair of reflectors are interposed between heat strips 80 and the outer shell of cover 76. The reflectors are, preferably, made from aluminum, and are secured to the insulation disposed on the outer shell of cover member 76. Lower housing member 78 defines an open ended chamber having transport means 82 disposed therein for advancing support material 34. Transport means 82 includes a pair of spaced rollers 84 and 86 mounted rotatably on the frame of the fusing device. An endless belt 88 having a plurality of apertures 90 therein is entrained about rollers 84 and 86. Air is drawn through apertures 90 of belt 88 so as to tack support material 34 thereto as it passes through fusing device 56. Preferably, endless belt 88 is made from a high temperature elastomeric material such as silicone rubber. However, any flexible material having high thermal resistivity may be suitable for the foregoing purpose. As shown in FIG. 2, thermal protector 58 includes detecting means or lever arm adapted to engage the underside 88a of endless belt 88 and de-energizing means or switch 1 12. Switch 1 12 is arranged to be in the closed position when endless belt 88 is as shown in- FIG. 2. When switch 112 is in the closed position the printing machine and fusing device 56 may be actuated. However, should switch 112 be in the open position, the printing machine and fusing device 56 will be de-energized.

While the present invention has been described in connection with switch 112 being disposed beneath underside 88a of endless belt 88, one skilled in the art will appreciate that the invention is not necessarily so limited and that switch 112 may be located remotely from underside 88a of endless belt 88. In this case, a lever/- rod system is connected to switch 112 and engages underside 88a of endless belt 88. This permits switch 1 12 to be located in a cooler location increasing the life expectancy thereof. Moreover, by a judicious selection of lever arm lengths, the upward force exerted on underside 88a of endless belt 88 may be minimized.

Turning now to FIG. 3, there is shown a plan view of lower housing assembly 78. Lower housing assembly 78 includes a sheet metal shell 94 having insulation secured to the interior surface thereof. Transport frame 96 is mounted removably in shell 94. Rollers 84 and 86 are rotatably mounted on frame 96 and have entrained thereabout endless belt 88. Interior surface 88b of endless belt 88 is closely adjacent to plate member 98. Plate member 98 is heated by the flow of warm air in the direction of arrows 102. Blower member 104 rotates vane member 106 such that air flows in the direction of arrows 102. The air flow passes over heating means or auxiliary heater 110 onto plate member 98 raising the temperature thereof. Plate member 98 transmits heat to endless belt 88. This, in turn, raises the temperature of support material 34 minimizing any heat loss thereto. In this manner, radiant energy from heat strips 80, in conjunction with heated support material 34 coalesce the multi-layered toner powder image formed thereon. When the heating elements of fusing device 56 are operating satisfactorily and endless belt 88 is moving, lever arm 110 engages surface 88 a of endless belt 88 and switch 112 remains in the closed position. However, in the event of a malfunction, i.e., endless belt 88 ceases to move, lever arm 1 will move switch 112 to the open position deactuating the printing machine. As described herein, switch 112 is adapted to de-energize the printing machine. However, in liew thereof, one skilled in the art will readily appreciate that switch 112 may be designed to de-actuate heat strips 80 alone, or heater 108 and heat strips 80, or auxiliary heater 108 alone.

With continued reference to the drawings, FIG. 4 illustrates a sectional elevational view of the air flow in lower housing 78. As shown therein, air moves downwardly through the exterior surface 88a of endless belt 88 in the direction of arrows 102. Air passes through the'open ends of endless belt 88, as indicated by arrows 102, and over the heating elements of auxiliary heater 108. Air is drawn into vane member 106 onto plate member 98 adjacent interior surface 88b of endless belt 88. Heating means 108 is, preferably, an 800 watt tubular high mass heater. Once again, thermal protector 58 is shown therein with lever arm 110 engaging undersurface 88a of endless belt 88. Thermal protector 58 will be further described hereinafter with reference to FIGS.- 50 and 5b.

Turning now to FIG. 5a, thermal protector 58 is shown therein with switch 112 in the closed position. As depicted therein, lever arm actuator 110 is adapted to engage undersurface 88a of endless belt 88. Lever arm 110 is mounted on switch 112. Roller 100 is mounted rotatably on lever arm 110 at the end thereof in engagement with undersurface 88a of endless belt 88. Switch 112 is inserted beneath frame 96 which includes an aperture therein permitting lever arm 110 to pass therethrough. Switch 112 resiliently urges lever arm 110 in an upwardly direction so that roller 100 thereof engages underside 88a of endless belt 88. When lever arm 110 is depressed, switch 112 is in the closed position. Thus, when endless belt 88 is in the unblistered state it maintains lever arm 110 depressed and switch 112 closed. If, however, endless belt 88 should remain stationary, the radiant energy from heat strips 80 will blister or lift the belt in an upwardly direction from the surface of frame 96. If the heat blister formed in endless belt 88 is approximately 0.100 inches, lever arm 110 will pivot in an upwardly direction opening switch 112 and de-energizing the printing machine. Switch 112 normally remains in the open position, unless depressed in a downwardly direction to the closed position thereof. In operation, endless belt 88 exerts a downward force on roller 100 of lever arm 110, thereby depressing switch 112 to the closed position. Switch 112 is connected in a low voltage circuit controlling a relay interposed between the printing machine external voltage excitation source (not shown) and the printing machine. Thus, when endless belt 88 forms a heat blister therein, it no longer exerts a downward force on roller 100 of lever arm 110. Thereupon, lever arm 110 pivots in an upwardly direction opening switch 112 and deactivating the relay, thereby deenergizing the printing machine.

Endless belt 88 is shown in FIG. 5b in a blistered condition. As shown therein, blister 116 permits lever arm to pivot to the open position, thereby opening switch 112 and de-energizing the printing machine as hereinbefore described.

Another feature of the present invention is the utilization of a thermal fuse, i.e., a buss heat limiter, connected in series with switch 112. The thermal fuse provides additional protection insuring printing machine shut-down in all over temperature conditions.

By way of example, switch 112 may be a single pole, double throw microswitch excitable at 115 volts d.c. The maximum operating force is, preferably, about 7 ounces, the maximum release force being about 1 ounce. Furthermore, the upper maximum operating temperature is about 500 F. In operation, switch ll2is designed to de-energize the printing machine when endless belt 88 blisters, as shown in FIG. 5b. Blistering will occur, for a silicone elastomeric material, at about 500 F. In addition, if endless belt 88 blisters, support material 34 will most likely have reached its kindling temperature and flame. Thus, in the foregoing arrangement not only is the fusing device protected from a catastrophic failure, i.e., endless belt 88 would be totally destroyed if it blistered extensively, but support material 34 is prevented from burning. It should be noted that by de-energizing the printing machine as endless belt 88 starts to blister, endless belt 88 will not be destroyed but will return to its original configuration and may be reused thereafter.

In recapitulation, the thermal protector employed in the fusing device of the electrophotographic printing machine is adapted to indicate when the belt thereof starts to blister. This is achieved by a lever arm engaging the undersurface of the belt. The lever arm is arranged to maintain the switch associated therewith in the closed position. However, if the belt starts to blister, the lever arm pivots, and the switch opens, thereby de-energizing the printing machine and preventing a catastrophic fuser failure and/or a fire therein.

Thus, it is apparent that there has been provided, in accordance with this invention, a thermal protector for the fusing device of an electrophotographic printing machine that fully satisfies the objects, aims and advantages set forth above. While this invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.

What is claimed is:

1. An apparatus for preventing an over-temperature condition in a fusing device having an article passageway therethrough, including:

means for transporting the article along a path of movement arranged to pass through the passageway of the fusing device; and

means, arranged substantially in contact with the surface of said transport means opposed from the article supporting surface thereof, for detecting the formation of a heat blister in said transport means indicative of an over-temperature condition therein.

2. An apparatus as recited in claim 1, further including means, responsive to said detecting means indicating the formation of a heat blister in said transport means, for de-energizing the fusing device.v

3. An apparatus as recited in claim 1, wherein said transport means includes an endless belt entrained about a plurality of space opposed rollers.

4. An apparatus as recited in claim 3, wherein:

said de-energizing means includes switch means arranged to deactuate the fusing device when said endless belt forms a heat blister therein; and

said detecting means includes a lever arm having one end portion thereof mounted on said switch means and the other end portion thereof positioned in contact with the surface of said endless belt opposed from the article supporting surface thereof, said lever arm being depressed by said endless belt to maintain said switch means in the closed position and being adapted to pivot in the presence of a heat blister in said endless belt to move said switch means to the open position deactuating the fusing device.

5. An electrostatographic printing machine of the type having a powder pattern formed on a sheet of support material including:

'a housing member having a passageway therethrough;

means for transporting the sheet of support material along a path of movement arranged to pass through the passageway of said housing member;

means for heating said transport means;

a radiant energy source arranged to be in thermal communication with the sheet of support material for supplying the energy output thereof onto the powder pattern formed thereon for substantially permanently affixing the powder pattern thereto; and

means, arranged substantially in contact with the surface of said transport means opposed from the sheet supporting surface thereof, for detecting the formation of a blister in said transport means indicative of an over-temperature condition therein.

6. A printing machine as recited in claim 5, further including means, responsive to said detecting means indicating the formation of a blister in said transport means, for de-energizing the printing machine.

7. A printing machine as recited in claim 6, wherein said transport means includes:

a lower housing member defining an interior open ended chamber;

a frame member mounted removably in the chamber of said lower housing member;

a plurality of rollers mounted rotatably on said frame member, said rollers being positioned spaced from one another and having the axis of rotation thereof substantially parallel to one another;

an endless belt having a plurality of apertures therein, said endless belt being entrained about said rollers; and

means for removing air from the chamber of said lower housing to secure releasably the sheet of support material to the surface of said endless belt.

8. A printing machine as recited in claim 7, wherein:

said de-energizing means includes switch means arranged to deactuate the printing machine when said endless belt forms a heat blister therein; and

said detecting means includes a lever arm having one end portion thereof mounted on said switch means and the other end portion thereof positioned in contact with the surface of said endless belt opposed from the sheet supporting surface thereof, said lever arm being depressed by said endless belt to maintain said switch means in the closed position and being adapted to pivot in the presence of a heat blister on said endless belt to move said switch means to the open position deactuating the printing machine.

9. A printing machine as recited in claim 8, wherein said heating means includes:

a plate member mounted in the open end of the chamber of said lower housing member interposed between said rollers and closely adjacent to the interior surface of said endless belt; and

at least one resistance heating element disposed in the chamber of said lower housing member, said heating element being positioned in the path of movement of the air being removed from the chamber heating the air which heats said endless belt.

10. A printing machine as recited in claim 9, wherein said radiant energy source includes:

a cover member mounted on said lower housing member and defining with said lower housing member the passageway enabling said endless belt to move the sheet of support material therethrough;

at least one radiant heat strip secured to said cover member, said heat strip being configured to furnish substantial uniform radiation across the surface of the sheet of support material; and

reflecting means interposed between said cover member and said radiant strip directing the energy output therefrom onto the sheet of support material to coalesce and permanently affix the powder pattern thereto. 

1. An apparatus for preventing an over-temperature condition in a fusing device having an article passageway therethrough, including: means for transporting the article along a path of movement arranged to pass through the passageway of the fusing device; and means, arranged substantially in contact with the surface of said transport means opposed from the article supporting surface thereof, for detecting the formation of a heat blister in said transport means indicative of an over-temperature condition therein.
 2. An apparatus as recited in claim 1, further including means, responsive to said detecting means indicating the formation of a heat blister in said transport means, for de-energizing the fusing device.
 3. An apparatus as recited in claim 1, wherein said transport means includes an endless belt entrained about a plurality of space opposed rollers.
 4. An apparatus as recited in claim 3, wherein: said de-energizing means includes switch means arranged to deactuate the fusing device when said endless belt forms a heat blister therein; and said detecting means includes a lever arm having one end portion thereof mounted on said switch means and the other end portion thereof positioned in contact with the surface of said endless belt opposed from the article supporting surface thereof, said lever arm being depressed by said endless belt to maintain said switch means in the closed position and being adapted to pivot in the presence of a heat blister in said endless belt to move said switch means to the open position deactuating the fusing device.
 5. An electrostatographic printing machine of the type having a powder pattern formed on a sheet of support material including: a housing member having a passageway therethrough; means for transporting the sheet of support material along a path of movement arranged to pass through the passageway of said housing member; means for heating said transport means; a radiant energy source arranged to be in thermal communication with the sheet of support material for supplying the energy output thereof onto the powder pattern formed thereon for substantially permanently affixing the powder pattern thereto; and means, arranged substantially in contact with the surface of said transport means opposed from the sheet supporting surface thereof, for detecting the formation of a blister in said transport means indicative of an over-temperature condition therein.
 6. A printing machine as recited in claim 5, further including means, responsive to said detecting means indicating the formation of a blister in said transport means, for de-energizing the printing machine.
 7. A printing machine as recited in claim 6, wherein said transport means includes: a lower housing member defining an interior open ended chamber; a frame member mounted removably in the chamber of said lower housing member; a plurality of rollers mounted rotatably on said frame member, said rollers being positioned spaced from one another and having the axis of rotation thereof substantially parallel to one another; an endless belt having a plurality of apertures therein, said endless belt being entrained about said rollers; and means for removing air from the chamber of said lower housing to secure releasably the sheet of support material to the surface of said endless belt.
 8. A printing machine as recited in claim 7, wherein: said de-energizing means includes switch means arranged to deactuate the printing machine when said endless belt forms a heat blister therein; and said detecting means includes a lever arm having one end portion thereof mounted on said switch means and the other end portion thereof positioned in contact with the surface of said endless belt opposed from the sheet supporting surface thereof, said lever arm being depressed by said endless belt to mAintain said switch means in the closed position and being adapted to pivot in the presence of a heat blister on said endless belt to move said switch means to the open position deactuating the printing machine.
 9. A printing machine as recited in claim 8, wherein said heating means includes: a plate member mounted in the open end of the chamber of said lower housing member interposed between said rollers and closely adjacent to the interior surface of said endless belt; and at least one resistance heating element disposed in the chamber of said lower housing member, said heating element being positioned in the path of movement of the air being removed from the chamber heating the air which heats said endless belt.
 10. A printing machine as recited in claim 9, wherein said radiant energy source includes: a cover member mounted on said lower housing member and defining with said lower housing member the passageway enabling said endless belt to move the sheet of support material therethrough; at least one radiant heat strip secured to said cover member, said heat strip being configured to furnish substantial uniform radiation across the surface of the sheet of support material; and reflecting means interposed between said cover member and said radiant strip directing the energy output therefrom onto the sheet of support material to coalesce and permanently affix the powder pattern thereto. 